Internet Protocol Multimedia Subsystem (IMS) is an architectural framework defined by the 3rd Generation Partnership Project (3GPP) for delivering Internet Protocol (IP) multimedia to user equipment (UE) of the IMS network. An IMS core network (sometimes referred to as the “IMS core”, the “Core Network (CN),” or the “IM CN Subsystem”) permits wireless and wireline devices to access IP multimedia, messaging, and voice applications and services. IMS allows for peer-to-peer communications, as well as client-to-server communications over an IP-based network.
Before a UE can utilize IMS-based services, the UE performs a registration procedure, which involves transmitting registration requests to, and receiving registration responses from, the IMS core. During this registration procedure, if there is an issue with registering the UE for service, the IMS core may send the UE a registration response that specifies an error, such as a 403 Forbidden error. Pursuant to 3GPP standards, if the UE receives a 403 Forbidden response to a registration request, the UE is to re-attempt registration. However, this is a suboptimal approach to handling a 403 Forbidden registration response because it may be the case that the issue causing the registration failure will never be resolved, and the UE will never be registered for service, regardless of how many times the UE reattempts registration. Repeated attempts at registration place an undue burden on network resources, especially when many UE's are engaged in the same type of repeated registration reattempts.
Another approach to handling a 403 Forbidden response to a registration request is to refrain from re-attempting registration altogether if the UE receives such an error in a registration response. This is also suboptimal because it may prevent UEs from reattempting registration when these UEs would otherwise successfully register if registration was reattempted. Accordingly, at least some UEs will unduly go without service using this approach. The difficulty in devising a suitable approach to handling a 403 Forbidden response to a registration request lies in the inability of the UE and the IMS core to discern the exact issue that is causing the registration failure, as there may be many possible issues causing a 403 Forbidden registration response.